Zoom lens systems have been used in a variety of applications, such as image capture devices for capturing still images or moving images. Examples of still images may include static pictures of landscapes, wildlife, or sports. Examples of moving images may include motion pictures of movies shot on film and digital imaging means. Although zoom lens technologies are known for all of these various applications, differences in these applications may lead to variations in the development of zoom lens systems for different applications, such as combinations in power distributions with different configurations for each application. In other words, one zoom lens system may be more suitable than another for a certain application.
In order to achieve a well-balanced wide angle zoom lens, an optical system requires the characteristics of a wide angle lens at the wide angle zoom setting. The optical system would also have the characteristics of a lens with longer focal lengths, which covers narrower fields of view. Achieving all these characteristics requires appropriate power distributions and an appropriate zoom lens type, which can correct characteristic aberration changes during zooming and focusing.
Focus Breathing
By way of example, considerations that may be significant for capturing moving images may be negligible or non-existent for capturing still images. One such consideration may be the effect of focus breathing. When the focus of a lens system is adjusted, lens elements for focusing may move. This movement may result in a change in the total focal length of the lens system. As total focal length may be related to zooming, the field of view (or angle of view) may change, similar to a zooming effect. For instance, as one changes the focus back and forth between the foreground to the background, the field of view may change such that it appears to be “breathing” (or zooming in and out) during the focus adjustment in real-time. Although these changes in the field of view may actually result from adjustments in focus settings, the changes may appear to be results from adjustments in zoom settings, even when zoom settings have not been adjusted.
In an example from a cinematography application, such as a movie scene, one may want to change focus from one actor to another actor during a conversation in the same field of view without changing the field of view, i.e., without focus breathing. Significant changes in the field of view during multiple focus adjustments may be undesirably distracting at a wide field of view, which has a deep depth of field.
When capturing a particular still image, one may be concerned about using a particular frame for that particular still image capture, not about maintaining the same particular frame for the next still image capture. With respect to this particular still image capture, the next still image capture may be completely unrelated in its framing. In other words, unlike capturing moving images, capturing still images may generally involve little or no concern about maintaining the same field of view for different still image captures with different focus settings. Furthermore, focus breathing may be detected during focus changes in real-time capturing of motion pictures, but effects during real-time focus changes may be ignored or even often unnoticed when capturing still images. Accordingly, capturing still images may generally involve little or no concern related to focus breathing.
Even when capturing moving images, focus breathing may still be a minor or negligible concern in some applications. For instance, a user of an ordinary video camcorder may be satisfied with an image capture device having a simpler optical lens system that does not include such precise controls for field of view and focusing.
However, for cinematography, the change in field of view during focusing with a wide angle lens may not be negligible. The reason is the deep depth of field. Thus, for wide angle lenses and wide angle zoom lenses, it is recommended to correct focus breathing.
Lens Shade
In addition to focus breathing, cinematography applications may also involve other considerations. For example, in order to provide high-quality image capturing for professional-level motion pictures, it is generally desirable to remove or reduce unwanted imaging effects on the captured image. Lighting may be a crucial variable to control, and collecting light from unintended light sources with high intensity may lead to some generally undesirable effects, such as ghost images which are formed by total reflections within lens surfaces and multiple reflections among lens surfaces.
In order to limit incoming light into a camera lens system from unintended sources, such as unwanted sunlight and spotlight for illumination, a lens shade (e.g., a matte box) may be attached to the outer lens barrel at the front of a lens system in a cinematography application. If the lens shade is too short, unwanted light may enter the lens system. If the lens shade is too long, designated rays may be clipped, and the lens shade itself may interfere with the incoming rays to cause vignetting and corner illumination fall-off. A lens system with a fixed front lens group may lead to fewer or no adjustments of the lens shade size and/or position. A zoom lens system with a front lens group that moves during zoom or focus may require designing a lens shade with critical points of footprints of rays, and such designing may lead to a lens shade that is too small at longer focal length settings. Therefore, for applications in cinematography, it may be highly advantageous to employ a lens system with a fixed front lens group that is stationary during functions that may involve moving lens elements, such as zoom and focus. Conversely, it may be uncommon to practice applications in cinematography with a lens system having a moving front lens group.
Lens Speed
For cinematography applications, lens speed may be another common consideration. Lens speed can be correlated to the maximum aperture stops of a lens, which can be quantified in terms of an F-number. The diameter of the aperture stop and F-number are inversely related, so the maximum diameter of aperture stop would correspond to a minimum F-number. Also, a lens with a larger aperture would have a smaller F-number, and vice versa. For example, lens A having a larger maximum aperture (smaller minimum F-number) will be able to pass through more light or marginal rays to the film (or image sensor) than lens B having a smaller maximum aperture (larger minimum F-number). Lens A would enable a faster shutter speed than lens B. Therefore, lens A (with a smaller minimum F-number) would be “faster” than lens B (with a larger minimum F-number). In other words, a larger aperture leads to a “faster” lens.
A common concern in cinematography applications is shooting pictures under lower illumination environments. For such environments in dim lighting, higher lens speeds and shallow depth of field are generally preferred. For example, a lens of relatively high speed may have an F-number of 2.8 or smaller.
In some applications, it may be desirable to attain image captures with specific areas that are out-of-focus, or “bokeh.” Bokeh can occur in an image area that is outside the depth of field. Faster lenses can have a shallower depth of field, which can be useful for providing images with bokeh. Thus, a high-speed lens may be required to attain images with a desirable amount of bokeh.
A similar parameter used in cinematography is T-number T/#. T-number is like F-number but additionally adjusted for the amount of light transmitted through the lens in actual usage. For instance, at a given lens aperture, the T-number will equal the F-number if the lens has 100% transmission, i.e., no loss of light. However, as light passes through a lens, there is loss (e.g., through absorption by the glass materials). Therefore, the T-number will be larger than the F-number. For cinematography applications, a minimum T-number smaller than 2.8 may be preferred. The use of T-number is relatively uncommon outside of cinematography.
Wide Angle Zoom Lenses
There are many types of zoom lenses, and one general way to distinguish between them is by the field of view (FOV, also known as angle of view) and the zoom ratio that they provide. A wide angle zoom lens generally starts with a wide angle of view at the wide end of its zoom range, which may be utilized for the perspective effect and deeper depth of field. For instance, an exemplary wide angle zoom lens may have some FOV values of 70 degrees or more, but an exemplary telephoto zoom lens may start with some FOV values of 30 degrees or less at the wide end of its zoom range.
In addition to different FOV values, there are other general distinguishing points between wide angle zoom lenses and telephoto zoom lenses. As an example, wide angle zoom lenses are generally used in association with shooting an overview of a scene and utilizing perspective with deeper depth of field at the wide end. A wide angle zoom lens may also cover closer to the FOV covering standard to near a telephoto region having a shallower depth of field. But a telephoto zoom lens is generally used in association with a shallow depth of field, and the perspective is different from the perspective of wide angle zoom lenses throughout its zoom range.
Although there may be no specific technical definition of a wide angle zoom lens, still photographers may recognize that a focal length of smaller than 28 millimeters for 135 format (24×36 mm) is a wide angle lens. Its FOV is 75.38 degrees. If the FOV is wider than 84 degrees, it may be called a super wide angle lens. Wide angle zoom lenses may include the equivalent focal length of 28 millimeters of 135 format size at the wide end.
Due to the different visual effects provided by different focal length ranges of different zoom lenses, cinematography applications may employ a wide variety of zoom lenses for presenting images according to different purposes. For example, a wide angle zoom lens may be used for capturing scenes with wide angles of view, which may provide a sense of exaggerated distance between objects in the captured image. Also, a wide angle zoom lens may capture images with deep depths of field, especially at smaller aperture, so that most or all of the objects in the capture image are in focus.
Prior Art Zoom Lenses
Although zoom lens systems are known for many various applications, not all zoom lens systems are applicable for all these various applications. For example, a given zoom lens system may be particularly designed for a certain application, but not suitable for another application. Additionally, combining techniques of different zoom lens systems may involve complicated considerations and may not be simple to realize.
U.S. Pat. No. 3,771,853 to Nakamura discloses a 4-group zoom lens with an exemplary angle of view of 74 degrees. The power sequence of the 4 lens groups is negative-positive-negative-positive, or NPNP. All of the lens groups are movable during zoom, including the first (or front) lens group that has the front lens element. Due to this motion of the front lens element, using Nakamura's zoom lens system with a lens shade may require many adjustments of the lens shade size and/or position in order to compensate for the different positions of the moving front lens element, as discussed above. Such many adjustments may be time-consuming and cumbersome. This type of zoom lens may be wide angle or have a short focal length. Also, the exit pupil distance may be short in Nakamura's type of zoom lens. When applied with an image sensor for digital imaging, a zoom lens system with a short exit pupil distance may have undesired pixel vignetting. Accordingly, Nakamura's zoom lens design may be undesirable for use with a movable front lens element, which may introduce operational issues for photographers. The lens shade or matte box may need to move with front lens group motion.
U.S. Pat. No. 5,416,639 to Yamanashi demonstrates zoom lens teachings including wide angle zoom lens embodiments with 5 lens groups having a power sequence of NPNPP. The first and fifth lens groups are stationary during zoom. The number of degrees of freedom on zoom may be three for aberration corrections. The fifth lens group may operate as a field flattening lens, and it can lead to a zoom lens that operates closer to a telecentric optical system with given power distributions. Due to this telecentricity, the corner illumination may be decreased by the restrictions of the lens mount area, such as limitations of the mechanical mount and its diameter restrictions. The resulting corner illumination may be poor. Moreover, field curvature and distortion during zoom may not be corrected well. It may not be possible to compensate these aberrations through the use of aspheric surfaces because the zoom movements may affect changes in the aberrations. With these zoom lens teachings, the positions on entrance pupils may be long so that the diameter of the front lens element may become large at the “wide” zoom position.
A short paper entitled “A complete set of cinematographic zoom lenses and their fundamental design considerations,” by Zuegge et al. provides teachings for zoom lenses for cinematography applications. Zuegge et al. mentions that its zoom lenses are for cinematography and have high performance that is as good as fixed focal length lenses with smaller aperture relative to conventional zoom lenses. The size and weight issues of a zoom lens are pointed out. The basic zoom type is PNP and, for wide angle zoom, the first group is split into two subgroups and the second positive subgroup is used for focusing. For long focal length zoom lenses, the focusing lens employs two separated groups which can be called internal focusing. Although overall lengths of these zoom lenses are fixed and they have internal focusing, the zoom ratio is approximately two.